1. Field of the Invention
This invention relates to a switching circuit device using current mirror circuits, and more particularly it pertains to such a switching circuit device which is operable with a low supply voltage and capable of selecting a desired one of a plurality of input signals, thereby achieving switching control of analog signals.
2. Description of the Prior Art
To have a better understanding of the present invention, description will first be made of a conventional switching circuit device illustrated in FIG. 1, wherein differential amplifiers which correspond in number to input signals, are provided, and a current source transistor of each differential amplifier is controlled to select a desired signal.
More specifically, the conventional switching device includes a first differential amplifier circuit A1 comprising a differential pair of transistors Q100 and Q101, an active load circuit constituted by a diode D100 and transistor Q102, and a feedback transistor Q103 and current source transistor Q104; and a second differential amplifier circuit A2 comprising transistors Q105 to Q109 and a diode D101 which are connected in a fashion similar to the first differential amplifier A1. The differential amplifiers A1 and A2 have their output terminals coupled to a common output terminal IOUT. The transistors Q100 and Q105 have their bases connected to input terminals IIN1 and IIn2 respectively. To achieve selection of a desired input signal, the current source transistor Q104 or Q109 is controlled so that the associated one of the differential amplifiers A1 and A2 is operated, thereby selecting the desired input signal.
However, the above-mentioned conventional switching circuit device is disadvantageous in that the number of components is increased due to the fact that it is constituted by using differential amplifiers. Another disadvantage is such that when the foregoing arrangement is provided in the form of a semiconductor integrated circuit, an increased area is required by the signal change-over portion thereof which is adapted for selecting one input signal out of more than two input signals in an alternative way, and obviously this is not preferable from the standpoint of circuit integration. Still another disadvantage is such that because of the fact that differential amplifiers are used and signal selection is effected by controlling the current source transistor of each differential amplifier, a large voltage drop is caused to occur in the differential amplifier so that with a power source voltage as low as about 1 volt, stable opening and closing operation is difficult to achieve.